Method and apparatus for feeding electrical current into an electrical power supply system

ABSTRACT

The invention relates to a method for feeding electrical current into an electrical, three-phase power supply system having a first phase, a second phase and a third phase with a first voltage, a second voltage and a third voltage at a power supply system frequency, comprising the steps of: measuring the first, second and third voltages, transforming the first, second and third voltages into a positive phase-sequence voltage system and a negative phase-sequence voltage system according to the method of symmetrical components, calculating a first desired current, a second desired current and a third desired current for feeding into the first, second and third phases of the power supply system, wherein the first, second and third desired currents are calculated on the basis of at least one value of the positive phase-sequence voltage system and/or the negative phase-sequence voltage system.

BACKGROUND

1. Technical Field

The present invention concerns a method and a apparatus for feedingelectric current into an electric three-phase network. The presentinvention also concerns a wind power installation which is adapted tofeed electric current into a three-phase network.

2. Description of the Related Art

Methods and apparatuses for feeding electric current into an electricthree-phase network such as for example the European integrated networkor into a part thereof are generally known. Large-scale power stationsuse for that purpose a synchronous generator connected directly to theelectric network. For that purpose the synchronous generator isoperating at a suitable rotary speed which is precisely matched to thefrequency of the electric network. Depending on the respectivestructural configuration of the synchronous generator the rotary speedis for example 1500 revolutions per minute in the case of a four-polesynchronous generator connected to a network involving a frequency of 50Hz. If disturbances occur in the network such as for example anasymmetric loading on the network in which for example one of the threenetwork phases is more heavily loaded, that has a direct effect on thecurrent delivered by the synchronous generator. In that case thephysically governed characteristics of the synchronous generator can atleast partially contribute to making the network symmetrical again. Thenature and fashion of such a contribution from the synchronous generatorhowever cannot basically be influenced because of the rigid coupling tothe network.

Wind power installations were still connected passively to the networkin the 1990's in the sense that they feed as much energy into thenetwork as is possible at the respective current moment in time inconsideration of the prevailing wind conditions. It was recognized forthe first time at the end of the 1990's that wind power installationscan also make a contribution to electrically supporting the network.Thus for example German patent application DE 100 22 974 A1 describes amethod in which wind power installations can change and in particularthrottle their feed of power into the network in dependence on thenetwork frequency. DE 101 19 624 A1 proposes that in the event of anetwork disturbance, more specifically in particular in the case of ashort-circuit, a wind power installation restricts the current which itfeeds into the network instead of being disconnected from the network inorder also thereby to achieve network support. WO 02/086315 A1 describesa method for network support by a wind power installation, which adjustsa phase angle of the fed-in current in dependence on the network voltageand thus feeds apparent power into the network in dependence on voltagein order thereby to support the network. DE 197 56 777 A1 also concernsa method of network support by means of a wind power installation, inwhich the wind power installation possibly reduces the power to be fedinto the network, in dependence on the network voltage, in order therebyin particular to avoid disconnection from the network, in order alsothereby to achieve support for the network by a wind power installation.

Wind power installations are becoming increasingly important. They arealso increasingly gaining in importance in regard to network support.The above-described network support measures—which can also be referredto as pioneering steps—are however still capable in that respect ofbeing improved in so far as network asymmetries are taken into account.

In that respect taking account of network asymmetries raises someproblems. Firstly problems are involved in rapidly and accuratelydetecting network asymmetries. In addition in the case of detection ofasymmetries, the problem arises of targetedly compensating for same,which is not possible or is only limitedly possible with a stronglycoupled synchronous generator. The same problems occur in systems whichdo not use a synchronous generator but simulate same in terms ofbehavior.

As state of the art attention is also to be directed generally to WO2010/028689 A1 relating to a wind power installation with a double-fedasynchronous machine.

BRIEF SUMMARY

One or more embodiments may be directed to resolving or reducing atleast one of the above-mentioned problems. One embodiment seeks toprovide a solution in which current is to be fed deliberatelyasymmetrically into the network in order to counter asymmetries presentin the network. The invention seeks at least to provide an alternativesolution.

According to one embodiment of the invention there is proposed a methodaccording to claim 1.

Accordingly there is proposed a method of feeding electric current intoan electric three-phase network having a first, a second and a thirdphase with a first, second and third voltage at a network frequency. Inthat respect the method is based on a three-phase system involving afrequency, namely the network frequency, in which each phase has its ownvoltage which can differ from the voltages of the other phases. Themethod therefore takes account in particular also of an asymmetricthree-phase system.

In a step the first, second and third voltages are measured and thevoltages are transformed into a voltage positive sequence and a voltagenegative sequence using the method of symmetrical components. Thus, thethree-phase voltage system can be described in a simple and generalfashion in spite of taking account of asymmetries. It is assumed thatonly three lines which are usually referred to as L1, L2 and L3 carrycurrent and thus a zero sequence is not present or is not needed fordescription purposes, but a description by voltage positive sequence andvoltage negative sequence is sufficient.

The method further involves calculation of a first, second and thirdtarget current for feeding into the first, second and third phaserespectively of the network. It is pointed out that the provision andfeed of such a first, second and third current—the three currents canalso be referred to together as a three-phase current—differsfundamentally and substantially from the generation of a three-phasecurrent for actuation of a device like an electric motor. Thus, when acurrent is fed into an electric network, there is usually not a directand in particular deterministic reaction to the feed into the network,as would be the case in the event of a well-known consumer. Admittedlythe electric network also reacts to the current which is respectivelyfed in, but nonetheless such a reaction is not comparable to that of adirectly present and clearly identifiable consumer such as for examplean electric motor.

Calculation of the first, second and third target currents is effectedin dependence on at least one value of the voltage positive sequenceand/or the voltage negative sequence. Thus it is firstly proposed thatin the event of a feed into the three-phase network asymmetries of thenetwork are to be taken into consideration and the three currents to befed in are to be correspondingly calculated. Therefore, to take accountof the network asymmetries, it is proposed that the target currents becalculated in dependence on the voltage positive sequence and thevoltage negative sequence respectively. In that way it is possible toreact in targeted fashion to corresponding asymmetries in the network.

In contrast to conventional large-scale power stations which provide forthe network feed by way of a star with a synchronous generator coupledto the network there is now proposed specifically targeted calculationof the target currents in dependence on the asymmetry or taking sameinto consideration.

This therefore involves taking account of any asymmetries of the networkvoltage, which has consequences in terms of calculation of the currentto be fed. Thus the positive sequence and/or negative sequence of thenetwork voltage acts on the currents to be fed in.

In an embodiment it is proposed that electric currents are produced bymeans of a frequency converter corresponding to the first, second andthird target currents for feeding into the three-phase voltage networkand are fed thereinto. That therefore basically directly involvesproduction of the currents by a converter, as can be implemented forexample by pulse width modulation. For that purpose the energy to be fedinto the network can be provided in a DC voltage intermediate circuitfrom which pulse width modulation is implemented in order torespectively generate an oscillating, in particular sinusoidal currentfrom the DC voltage signal of the DC voltage intermediate circuit.

Preferably for that purpose, in particular for the DC voltageintermediate circuit specified by way of example, the electric energy isprovided by a wind power installation and the AC voltage energy producedis converted by means of a rectifier into energy with a DC voltage. Thatis intended in particular to make it possible to use wind powerinstallations or wind parks with a plurality of wind power installationsfor stabilizing the network and in particular for stabilizing anasymmetric network. At least preferably the electric energy of windpower installations is fed into the network in such a way that anyasymmetry involved is not increased and the network condition istherefore not worsened.

In an embodiment the method is characterized in that to calculate thetarget currents a calculation phase angle is adopted as the basis andthe calculation phase angle is determined in dependence on a detectionof a network fault, in particular using a determination filter or afilter block. In that case the calculation phase angle is determinedfrom a detected phase angle of one of the network voltages if no networkfault was detected. Otherwise, if a network fault was detected or is tobe assumed, it is proposed that the calculation phase angle isdetermined in another way, in particular from a phase angle of thevoltage positive sequence, and/or that the calculation phase angle isdetermined using a predetermined network frequency.

Accordingly determination or calculation of the target currents is notdirectly based on a phase angle detected upon measurement of thethree-phase voltage, but a specific phase angle is calculated, whichforms the basis for calculation of the target currents and which forthat reason is identified as the calculation phase angle. Thecalculation phase angle should be distinguished for example by a highlevel of accuracy and/or low noise. The calculation phase angle can bedetermined for example by way of a determination filter or filter block.That determination filter or filter block can be for example in the formof a state observer. Calculation of the phase angle can be effected forexample in the manner described in German laid-open application DE 102009 031 017 A1 in connection with FIG. 4 therein. In particular theoperation of determining the currents can be effected in the way thatdetermination of the phase angle φ₁ from the detected phase angle φ_(N)is described therein.

The calculation phase angle is preferably determined from a phase angleof the voltage positive sequence if a network fault was detected. Herein particular the arrangement is switched over to that other source fordetermining the phase angle, which can also be implemented in the formof a software solution. The use of the phase angle of the voltagepositive sequence is proposed for that purpose. At least at thebeginning of a network fault which occurs it may be possible for thephase angle of the voltage positive sequence to deliver a reliable valueor an at least sufficiently reliable value for the phase angle.Frequently a network fault also involves problems in terms of voltagemeasurement. The reason for that can be for example that the voltagecannot be measured, it can be poorly measured or in part it cannot bemeasured or can be poorly measured. Another problem can be thatmeasurement or detection of the voltage of the three-phase system isbased on conditions which possibly no longer prevail in the networkfault situation.

Alternatively or additionally it is proposed that in the case of anetwork fault a predetermined network frequency is used for calculatingthe calculation phase angle. In the simplest case a fixed frequency suchas for example the nominal frequency, that is to say for example exactly50 Hz or exactly 60 Hz is constantly predetermined and the phase angleis determined therefrom, in the simplest case by way of integration.Preferably the use of a predetermined frequency is combined with the useof a phase angle from the voltage positive sequence. Such a combinationcan be effected in such a way that the operation of determining thevoltage positive sequence and in that case also the voltage negativesequence itself uses a predetermined frequency, in which case the phaseangle of the voltage positive sequence is correspondingly alsodetermined and is thus determined using the predetermined frequency.

It is further preferably proposed that for calculating the targetcurrents, in particular if a network fault is detected, reference ismade to the voltage positive sequence, in particular to the phase angleof the voltage positive sequence. An important aspect when feedingelectric current into an AC voltage network, in particular into athree-phase AC voltage network is the phase angle with which it is fedinto the network. Predetermining an phase angle of the three-phasetarget current or a phase angle for each target current presupposesknowledge which is as precise as possible of the network phase angle orknowledge which is as precise as possible is desirable. Therefore aproblem in terms of feeding the three-phase current into the network isimprecise knowledge of the network phase angle, which for example canoccur when there is a network fault. Equally—possibly with theadditional problem of inaccurate measurement—an asymmetric network isproblematical because problems can already occur in establishing asuitable phase angle for such an asymmetric situation.

Referencing to the phase angle of the voltage positive sequencecomponent proposes here a solution which addresses those problems.Determining the positive sequence component which includes the step ofdetermining the phase angle of the positive sequence component affords acomparatively stable value which at the same time takes account of anyasymmetries of the three-phase network. Referencing of the operation ofdetermining the target currents thereto, that is to say taking the phaseangle of the positive sequence or a phase angle of the voltage ascalculated therefrom as the basis, thus permits suitable target currentpresetting even for non-ideal conditions in the three-phase network.

Particularly if there are ideal conditions in the three-phase network,it is possible to switch over to using the detected phase angle of oneof the network voltages and vice-versa. Switching over in that way ispreferably effected at the input side to a filter, in particular at theinput side of the determination filter or the filter block. Acorresponding switching-over jump can also be filtered by using adetermination filter or filter block. If for example a calculation phaseangle is determined from the detected phase angle of a network voltageor a phase angle of the voltage positive sequence, as is described inFIG. 4 of German laid-open application DE 10 2009 031 017 A1 then thedetermination filter or filter block has a second-order transfercharacteristic. A switching-over pulse or jump at the input of such adetermination filter has correspondingly slight effects at the output ofthe filter or filter block and thus there are only few or negligibleproblems upon further use for referencing purposes in calculating thetarget currents.

As a further embodiment there is proposed a method characterized in thattransformation of the first, second and third voltages into a voltagepositive sequence and a voltage negative sequence includestransformation of the first, second and third voltages by means of adiscrete Fourier transformation (DFT), wherein in particular thediscrete Fourier transformation is effected online and only over half aperiod duration. From the measured voltage values of the three phasesthe discrete Fourier transformation determines complex voltage valuesfor the three-phases, that is to say a voltage in respect of amplitudeand phase for each of the three phase voltages. To be able also to takeaccount of non-ideal conditions of the three-phase network, very fastdetection of the network situation, in particular fast detection ofchanges in the voltages in the network, can sometimes be important oreven of crucial significance for matched current feed into the network.When using the positive sequence component and the negative sequencecomponent, in particular upon referencing to the phase angle of thepositive sequence component of the voltage, a crucial change in thenetwork state should also be reflected in those components as quickly aspossible. Accordingly the discrete Fourier transformation should alsowork as quickly as possible.

Usually a Fourier transformation and thus also a discrete Fouriertransformation is based on at least one entire period duration. Thatforms the underlying basis and is also essential for correctimplementation of a Fourier transformation. It was however now realizedthat it may be sufficient to base it on half a period duration.Accordingly the Fourier transformation, namely the discrete Fouriertransformation, was adapted thereto.

Preferably the transformation is effected online, namely in the sensethat at each measurement point the values of the three voltages arerecorded and pass into the discrete Fourier transformation which is alsoperformed at each measurement time. Thus recorded measurement valuesalso act immediately on the result of the discrete Fouriertransformation. The respective currently measured measurement values areincorporated as new values and the remaining, already previouslymeasured values of the current half-wave are also involved. A change inthe situation in the network will thus have first effects with the firstmeasurement value, after measurement of half a period duration they willhave acted completely on the result of the discrete Fouriertransformation.

Thus a discrete Fourier transformation for half a period duration meansthat the respective current measurement values are recorded in themanner of a sliding value as far as the measurement values which areback by half a period duration, and they are incorporated into thediscrete Fourier transformation step.

Thus the duration, after which new measurement values act completely onthe result of the discrete Fourier transformation, can be halved inrelation to a conventional discrete Fourier transformation over anentire period length. Accordingly the discrete Fourier transformationwill lead to a result twice as quickly or any detection times can behalved.

In an embodiment there is proposed a method characterized in thattransformation of the first, second and third voltages into a voltagepositive sequence and a voltage negative sequence uses a predeterminedfrequency instead of measurement of a currently prevailing networkfrequency. Such a predetermined frequency can be for example the nominalfrequency of the network, therefore in particular 50 Hz in the case ofthe European integrated network or for example 60 Hz in the USA. Thepredetermined frequency can however also be established in some otherfashion, either as another fixed value or by a calculation specificationor the network frequency used is taken from a model.

This embodiment is based on the notion that the transformation can beinfluenced, in particular improved, in particular in the sense ofstabilizing the transformation, by presetting a network frequency. Sucha procedure can be used precisely when there is a network fault and theactual network frequency is difficult or inaccurate to measure or cannotbe measured at all.

Preferably the value of the network frequency of a measurement timewhich is further back can be used as the predetermined networkfrequency. In this case the transformation is oriented to the last, inparticular reliably measurable value of the actual network frequency.

A preferred configuration proposes that the method is characterized inthat the target currents are predetermined in accordance with the methodof symmetrical components by way of a current positive sequence and acurrent negative sequence. In particular in that respect the positivesequence is taken into account by a complex positive sequence currentcomponent in respect of amount and phase and the negative sequence istaken into account by a complex negative sequence current component inrespect of amount and phase.

The method of symmetrical components in known as the method of detectingan existing asymmetric three-phase system, that is to say it isbasically known as a measurement method. Here it is now proposed thatthe currents are to be predetermined based on analysis in accordancewith the method of symmetrical components. That presetting can beeffected in particular in such a way that two complex currents, namelythe positive sequence current component and the negative sequencecurrent component, are predetermined. On the basis thereof the threeindividual target currents are then respectively predetermined inrespect of amount and phase.

In an embodiment therefore calculation of the first, second and thirdcurrent target values is effected in dependence on a value of a voltagepositive sequence and/or a voltage negative sequence of the three-phasenetwork voltage present, wherein specific predetermination of thethree-phase current to be produced is effected by way ofpredetermination of positive sequence current components and negativesequence current components. This case is based on two completelydifferent procedures or steps.

In the first step the basic starting point is the actual state of athree-phase system, namely the three-phase voltage system, and thatactual state is reproduced using the method of symmetrical components.

The second step involves the target currents, wherein the procedureinvolved is entirely different, namely insofar as predetermination iseffected in the image domain. In particular a desired degree ofasymmetry can be predetermined by way of the positive sequence componentand the negative sequence component. Likewise the phase angle can bepredetermined in that image domain, that is to say the representationdomain, by the use of positive and negative sequence components. On thebasis thereof the actual target currents, that is to say the targetcurrents in the time domain, are then determined and finally suitablyconverted.

Preferably the target currents are predetermined by way of a positiveand negative sequence. Upon conversion of those target currents asindividual target currents in the time domain they are preferablyreferenced to a phase angle of the voltage positive sequence, that is tosay the positive sequence that is to be attributed to the actual stateof the voltages in the three-phase network.

Preferably the target currents are calculated in dependence on thecurrent positive sequence or the positive sequence current componentrespectively and when a network fault is assumed to occur they areadditionally calculated in dependence on the current negative sequenceor the negative sequence current component respectively. Thus inparticular a three-phase target current can be predetermined by way ofpositive sequence and negative sequence current components, whereas bothcomponents are used in dependence on the network situation, namely inthe case of a network fault, or only the positive sequence component isused if a network fault is not to be assumed. Network problems likenetwork faults and/or network asymmetries can be taken into accountthereby.

In that respect, particularly when there is a symmetrical fault-freenetwork, there is proposed an efficient feed method which in regard tothe target currents only takes account of the positive sequence currentcomponent which basically reflects the symmetrical network. If thenetwork is completely symmetrical in the mathematical sense thecounter-current component becomes zero and accordingly thecounter-current component will be small if slight asymmetries can beassumed to be involved. It is thus proposed that in suitable cases it ispossible to dispense with consideration of the counter-currentcomponent. For the sake of completeness it is pointed out that the termcounter-current component denotes the negative sequence currentcomponent and the term co-current denotes the positive sequence currentcomponent. The network can be identified as a voltage network toemphasize that the network operates on a voltage basis.

In a preferred embodiment the method is characterized in that thecurrent positive sequence or the positive sequence current component andthe current negative sequence or the negative sequence current componentare determined in dependence on predetermination of an active powercomponent of the positive sequence, a predetermination of a reactivepower component of the positive sequence and/or a predetermination of aquotient of the magnitude of the negative sequence current component inrelation to the magnitude of the positive sequence current component.

An active power component and a reactive power component can bepredetermined thereby in a simple fashion. Preferably thecounter-current component is used to counteract an asymmetry of theelectric three-phase network. Independently thereof an active andreactive power component of the current to be fed into the network canbe predetermined by way of the positive sequence current component. Thatis particularly advantageous and meaningful when the feed of the targetcurrents is referenced to the phase angle of the voltage positivesequence. In that way it is possible to feed substantially a symmetricalcurrent component into the network, adapted to the symmetrical voltagecomponent, and at the same time to take account of asymmetries both inthe detection operation and also in the feed into the network.

The operation of predetermining a degree of asymmetry or a parametercharacteristic in relation to a degree of asymmetry can be easilyeffected when predetermining the three-phase target current by way ofpositive and negative sequence components, if the quotient thereof,namely the quotient of negative sequence component to positive sequencecomponent, is established. Alternatively, instead of a fixed value it isalso possible to predetermine an upper limit for a degree of asymmetry.

Preferably the negative sequence current component is set and/or variedindependently of the positive sequence current component. Thus forexample firstly on the one hand the power which is substantially to befed into the network, in particular the active power, can bepredetermined in respect of amplitude by way of the positive sequencecurrent component. Accordingly in that way—expressed in simplifiedterms—the total current is predetermined in a first approximation inrespect of its amplitude. When predetermining the complex positivesequence current component division of active and reactive power oractive and reactive power component is also effected by way of the phaseangle, as was described above.

Firstly on the one hand an asymmetry can be predetermined by way of thecounter-current component. In particular it is possible to predeterminean asymmetry component in a specifically targeted fashion, in particularin respect of quality and quantity, for at least partially compensatingfor an asymmetry in electric voltage networks. Correspondingly the useof positive sequence current component and negative sequence currentcomponent affords a high degree of freedom in predetermining thethree-phase current to be fed into the network. The magnitude of thepositive sequence current component is in particular also substantiallyadjusted by the available power of the wind power installation and inthat respect in dependence on the prevailing wind conditions.

Preferably the method is carried out online. In particular preferablyall method steps are carried out online. It is possible in that way toreact as quickly as possible to any network changes and the feed of theelectric current can be suitably adapted. In particular the describedembodiments are adapted to such online implementation, as is the case inparticular for transformation of the detected three-phase voltage systeminto positive and negative voltage sequence components. In particularthe described discrete Fourier transformation which is adapted to theuse of only a half period duration permits such online implementation ofthe method of detecting and feeding the electric current.

According to one embodiment of the invention there is also proposed awind power installation which uses a method of feeding electric currentinto an electric three-phase voltage network of at least one of thedescribed embodiments.

According to one embodiment of the invention there is also proposed awind park comprising a plurality of such wind power installations. Sucha wind park, with modern wind power installations of today, can assumeorders of magnitude which permit a significant influence on the electricnetwork, in particular support for the electric network and also qualityimprovement of the current in the electric network.

In that respect the term wind park is used to denote an array of aplurality of wind power installations which interact with each other andin particular use one or more common feeding points for feeding electriccurrent into an electric network.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is described way of example hereinafter by means ofembodiments with reference to the accompanying Figures.

FIG. 1 shows a wind power installation,

FIG. 2 shows an overview diagram to illustrate an embodiment of themethod according to the invention, and

FIG. 3 shows in detail a calculation block of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a wind power installation which inter alia implements amethod according to one embodiment of the invention and for that purposehas at least one frequency converter with appropriate actuation in orderthereby to feed into an electric three-phase network.

The structure of an embodiment of the invention as shown in FIG. 2 isbased on a three-phase network 10 into which an inverter 12 feeds by wayof output chokes 14 and by way of a transformer 16. The transformer 16has a primary side 18 and a secondary side 20. The transformer 16 isconnected to the three-phase network 10 by way of a secondary side 20and the primary side 18 is connected to the inverter 12 by way of theoutput chokes 14.

The inverter 12 is supplied by way of a DC voltage intermediate circuit22 with a direct current or a DC voltage, preferably the DC voltageintermediate circuit 22 can be fed by a wind power installation or agenerator of a wind power installation, in which electric currentgenerated by the generator is rectified by means of a rectifier and fedinto that DC voltage intermediate circuit 22.

The inverter 12 thus produces from the direct current or the DC voltageof the DC voltage intermediate circuit 22 a three-phase alternatingcurrent which has three individual currents i₁, i₂ and i₃. Thethree-phase alternating current or the three individual currents areproduced by means of pulse width modulation. The pulses required forthat purpose are predetermined by means of a tolerance band control inaccordance with the corresponding tolerance band block 24. For thatpurpose the tolerance band block 24 receives the currents i₁, i₂ and i₃to be controlled, as reference or target values.

Calculation of the switching times based on the current target values iseffected by the tolerance band block 24 in basically known fashion inaccordance with a tolerance band method. In accordancetherewith—expressed in simplified terms—a switching operation of acorresponding semiconductor switch for producing or ending a pulse istriggered when a current actual value breaks out of a tolerance band,that is to say it exceeds the respective target value by more than atolerance value or falls below the respective current value by more thanthat or another tolerance value. In principle it is also possible to useother methods instead of this tolerance band method.

One aspect of the present invention is the operation of determiningthose target values i₁, i₂ and i₃ and/or the variation in the threetarget currents. In that respect the variation in the target currents isalso to be evaluated or interpreted in conjunction with the networkbehavior.

To feed current into the network as needed—instead of the term networkit is also possible to use the synonymous term grid network—thearrangement has the measurement filter 26 which measures the voltages ofthe three phases of the network and for that purpose has a respectivemeasurement resistor 28 and a measurement capacitance 30, that is to saya capacitor. Those components are provided for each phase and as shownin FIG. 2 measurement of the voltages is effected at the primary side 18of the transformer 16. Alternatively, measurement can also beimplemented on the secondary side 20 of the transformer 16 or also atother locations in the network 10. For performing measurements inrespect of a three-phase network attention is also directed to Germanpatent application DE 10 2009 031 017 A1 which discloses in FIG. 3thereof a measurement filter corresponding to the measurement filter 26of this FIG. 2.

The measured voltages v_(L1)(t), v_(L2)(t) and v_(L3)(t) are inputtedinto the transformation block 32 which calculates a transformation at atime of the voltage values measured in polar co-ordinates into acomplex-value parameter with magnitude and phase, namely the networkvoltage V_(N) as magnitude and the angle φ_(N) as phase. The phase anglerelates to the first voltage. The calculation operation can be carriedout as follows, wherein v₁, v₂ and v₃ respectively represent theinstantaneous voltage value of the voltages v_(L1)(t), v_(L2)(t) andv_(L3)(t):

$\overset{arrow}{v} = \lbrack {v_{1} + {v_{2}{\exp( {j\frac{2}{3}\pi} )}} + {v_{3}{\exp( {j\frac{4}{3}\pi} )}}} \rbrack$$V_{N} = {\sqrt{\frac{2}{3}}\sqrt{( {{real}( \overset{arrow}{v} )} )^{2} + ( {{imag}( \overset{arrow}{v} )} )^{2}}}$$\varphi_{M} = {\arctan( {{{imag}( \overset{arrow}{v} )}/{{real}( \overset{arrow}{v} )}} )}$

Those equations and further description relating thereto are moreover tobe found in above-mentioned German laid-open application DE 10 2009 031017 A1.

The network voltage V_(N) determined in that way and the network phaseangle φ_(N) determined in that way are inputted into a state observerblock 34 which can also be referred as the SO1 block. The input of thestate observer block 34 for the phase angle also has a state switch 36which can be switched over in dependence on any fault situation in thenetwork in such a way that, instead of the network phase angle φ_(N), asthe output of the transformation block 32, another phase angle can beinputted into the state observer block 34, which will be furtherdescribed hereinafter.

The state observer block 34 outputs as the observed state parameter theestimated parameter V as an estimated effective value of the networkvoltage and the estimated phase angle φ as the estimated value of thenetwork phase angle.

A possible configuration of the transformation block 32 together withthe state observer block 34 can also be found in the above-mentionedGerman laid-open application DE 10 2009 031 017 A1. In that respectattention is directed to FIG. 4 together with the related description.The transformation block 32 can for example be of such a configurationas shown at block No 6 in FIG. 4 of that laid-open application. Thestate observer block 34 can be for example of a configuration as shownby block F1 with blocks 10 and 12.

The estimated phase angle φ is inputted directly into a decision block38. The decision block 38 calculates target values for the three-phasecurrents i₁(t), i₂(t) and i₃(t). Those target currents form the inputfor the tolerance band block 24 and thus the basis for modulation whichis carried out in the inverter 12. The estimated phase angle φ is animportant basic parameter for that purpose because a feed of alternatingcurrents into a running alternating current network is possible onlywith knowledge of the respective instantaneous phase angle in thenetwork. Nonetheless the decision block 38 takes account at leastindirectly of further items of information, namely the estimated networkvoltage V, any predetermination values of active and/or reactive powerto be fed into the network and the information as to whethera—relevant—network fault is present or could be present. Those items ofinformation ultimately pass by way of a PQ control block 40 into thedecision block 38. The decision block 38 performs calculation ordetermination of the target currents i₁(t), i₂(t) and i₃(t), wherein theunderlying calculation depends on whether a network fault was or was notdetected. For that reason the term decision block was also adopted forthat block 38. The internal calculations in the decision block 38 arealso set forth below. Further items of information relating to the PQcontrol block are also set forth hereinafter.

The decision block 38 uses—and this is also set forth in detailhereinafter—a breakdown into a positive sequence and a negativesequence. In corresponding fashion the positive sequence current or thepositive sequence current component I ⁺ and the negative sequencecurrent or the negative sequence current component I ⁻ respectivelyforms an input parameter of the decision block 38. The decision block 38is based—at any event if no network fault is present—on the positivesequence which generally in this application is characterized by asuperscript plus sign whereas the negative sequence component ischaracterized by a superscript minus sign. In other words the system inFIG. 2 and in particular the calculation in the decision block 38 isreferenced to the positive sequence component.

Breakdown of the measured voltages V₁(t), V₂(t) and V₃(t) into apositive sequence or a negative sequence is performed in the calculationblock 42 which for that purpose has a predetermined frequency f_(set).In the simplest case that frequency can be the assumed networkfrequency, that is to say for example 50 Hz in the European integratednetwork or 60 Hz in the USA network. It is however also possible toadopt other values, possibly also variable values.

In addition as input signals the decision block 38 receives at leastalso the phase angle φ _(Vfset) of the negative sequence in accordancewith the transformation of the three-phase voltage in the calculationblock 42. In addition the decision block 38 receives as input a flag asan indicator as to whether a network fault is or is not assumed to bepresent. The calculations performed in the decision block 38 in respectof the three target currents i₁(t), i₂(t) and i₃(t) are carried out independence on the value for the flag.

If the flag is 0, that is to say there is no fault situation, the threecurrents are calculated as follows:i ₁(t)=√{square root over (2)}I ⁺ cos(φ+φ _(I) ₊ )i ₂(t)=√{square root over (2)}I ⁺ cos(φ+φ _(I) ₊ +⅔π)i ₃(t)=√{square root over (2)}I ⁺ cos(φ+φ _(I) ₊ +4/3π)

The respective instantaneous value of the respective target current isthus based on the magnitude of the positive sequence target current I ⁺,the estimated network phase angle φ and the phase angle of the targetcurrent of the positive sequence φ _(I) ⁺ . The estimated network phaseangle φ specifies in that respect the respectively current absolutephase angle of the network voltage, with respect to the first phase. Thephase angle of the positive sequence current component φ _(I) ⁺specifies the phase angle of the current of the positive sequence inrelation to the phase angle φ of the network voltage.

If the flag assumes the value 1 (flag=1) it is assumed that there is anetwork fault. Such network faults or network disturbances include:

the loss of angle stability,

the occurrence of network islanding,

the occurrence of a three-phase short-circuit, and

the occurrence of a two-pole short-circuit.

Further information relating to the nature of such network disturbancesis also to be found in above-mentioned laid-open application DE 10 2009031 017 A1. The occurrence of such network faults can in particular alsohave the result that detected network states, in particular the phaseangle φ and the voltage level V were wrongly detected and/or areunsuitable for or are poorly suited to orientation for the currents tobe fed into the network. Calculation in the decision block 38 for thesituation where a network fault is assumed to occur is thusbased—speaking generally—more greatly on parameters ascertained in thecalculation block 42 and thus more greatly on the predeterminedfrequency f_(set). That is intended only to serve for generalexplanation and in that respect as a precaution it is pointed out thatcalculations in the calculation block 42 about determining the positivesequence component I ⁺ and the negative sequence component I ⁻ areinvolved in the decision block 38 and are thus also of relevance forcalculation in the decision block 38 without the assumption of a networkfault.

Upon the assumption of a network fault (flag=1), after calculation ordetermination of the target currents i₁(t), i₂(t) and i₃(t) thefollowing three steps are proposed. The following calculation steps—andalso the above-mentioned calculation in the situation without a networkfault—are effected for that time at which a respective instantaneousvalue is transferred to the tolerance band block 24 for the three targetcurrents i₁(t), i₂(t) and i₃(t).

In the first step a cos-component I ^(+c) and I ^(−c) and asin-component I ^(+s) and I ^(−s) are respectively calculated for thepositive sequence and the negative sequence as follows:i ^(+c)=√{square root over (2)}I ⁺ cos(φ+φ _(I) ₊ )i ^(+s)=√{square root over (2)}I ⁺ cos(φ+φ _(I) ₊ )i ^(−c)=√{square root over (2)}I ⁻ cos(φ _(V) ⁻ _(fset)+φ _(I) ⁻ )i ^(−s)=√{square root over (2)}(−I ⁻)sin(φ _(V) ⁻ _(fset)+φ _(I) ⁻ )

In the above equation system of the first step, I ⁺ denotes themagnitude of the positive sequence component and correspondingly I ⁻denotes the magnitude of the negative sequence component. φ _(I) ⁺ and φ_(I) ⁻ respectively denote the phase angle of the positive sequence andthe negative sequence respectively. In accordance with the FIG. 2structure those angles are not expressly passed to the decision block38, but are inherent elements of the complex positive sequence componentI ⁺ and the complex negative sequence component I ⁻. The phase angle ofthe negative system component of the voltage, as is determined in thecalculation block 42, which will be further described hereinafter, isdirectly passed to the decision block 38.

It is to be observed that a breakdown of the three-phase networkvoltages into a positive sequence component and a negative sequencecomponent is implemented in the calculation block 42, more specificallybased on the fundamentally known method of symmetrical components. Thatmethod of symmetrical components also forms the basis for the operationof determining a positive system component and a negative systemcomponent of the current in accordance with the PQ control block 40.Those two current components are passed as complex values to thedecision block 38. While that breakdown of the network voltage inaccordance with the calculation block 42 in the ideal case represents arepresentation of the actual state of the network voltages the divisioninto positive and negative sequences for the current in PQ control block40 includes a representation of the desired current to be fed into thenetwork or in preparation for the desired current to be fed into thenetwork. Thus that representation of the positive and the negativesequences for the current can include for example a desired phase shiftof the current relative to the voltage to feed a desired reactive powercomponent into the network.

Taking the cos- and sin-components calculated in that way for thepositive sequence and also for the negative sequence I ^(+c), I ^(−c), I^(+s), I ^(−s) an auxiliary current value i* and an auxiliary anglevalue φ* are now calculated in the second step as follows:

$i^{*} = \sqrt{( {i^{+ c} + i^{- c}} )^{2} + ( {i^{+ s} + i^{- s}} )^{2}}$$\varphi^{*} = {\arctan\lbrack \frac{i^{+ s} + i^{- s}}{i^{+ c} + i^{- c}} \rbrack}$

Finally in the third step for each of the target currents i₁(t), i₂(t)and i₃(t) a respective value for the moment in time in question iscalculated from the auxiliary current value i* and the auxiliary anglevalue φ*, as follows:i ₁(t)=i*cos(φ*)i ₂(t)=i*cos(φ*+⅔π)i ₃(t)=i*cos(φ*+4/3π)

It is to be noted that in this third step three individual values arecalculated for the three target currents i₁(t), i₂(t) and i₃(t). That iseffected for each calculation time, that is to say a plurality of timesfor each period duration. It is further to be noted that at each momentin time the auxiliary current value i* and the auxiliary angle value φ*change. Depending on the respective change in those values therefore theresult of the calculation of that step three must not lead to asymmetrical three-phase current system although the three equations ofthe calculation in step three differ only in an angle offset of ⅔π and4/3π respectively. Nonetheless therefore an asymmetric predeterminationof the three current and thus an asymmetric feed is just as possible asa symmetrical feed. The same moreover also applies in substance for theabove-represented calculation of the target currents i₁(t), i₂(t) andi₃(t). In the decision block 38 if a network fault is not assumed toapply, therefore if flag=0.

FIG. 3 shows details of the calculation block 42 of the overallstructure shown in FIG. 2. In accordance therewith the measured networkvoltages v₁(t), v₂(t) and v₃(t) are detected and therefrom transformedor converted into complex voltages V ₁, V ₂ and V ₃ in thetransformation block 50 identified as the half-cycle DFT. In the idealcase those complex voltages V ₁, V ₂ and V ₃ are only a differentrepresentation for the measured voltages v₁(t), v₂(t) and v₃(t) andpresuppose a sinusoidal configuration of a fixed frequency.

The three complex voltages V ₁, V, and V _(t) therefore define athree-phase voltage system which however can be asymmetric. Breakdown ofthat three-phase system is accordingly effected into positive sequencecomponent and a negative sequence component, based on the method ofsymmetrical components. The positive system component, namely itsmagnitude V⁺ _(fset) and phase φ_(V) ⁺ _(fset), is calculated in thepositive sequence transformation block 52, and the negative sequencecomponent, namely its magnitude V⁻ _(fset) and phase φ_(v) ⁻ _(fset) iscalculated in the negative sequence transformation block 54. Both thehalf-cycle DFT calculation block 50 which can also be simply identifiedas the DFT transformation block and also the positive sequencetransformation block 52 and the negative sequence transformation block54 use for their calculation a set frequency f_(set) which is inputtedfrom the exterior and the angle φ_(fset) calculated therefrom. Thepredetermined or fixed angle φ_(fset) is afforded by integration of thepredetermined or set frequency f_(set) in the integration block 56.

Calculation of a positive sequence which can also be referred to as apositive sequence component and a negative sequence which can also bereferred to as a negative sequence component is basically known from thetheory of the method of symmetrical components. In that respect anasymmetric three-phase system of so-called phasors is divided intopositive sequence, negative sequence and zero sequence. The positivesequence has the same direction of rotation as the underlyingthree-phase system whereas the negative sequence has an oppositedirection to that original system. The positive sequence considered initself and also the negative sequence considered in itself are eachsymmetrical in themselves. The zero sequence denotes a sequence in whichall phasors involve the same direction and the same length. That zerosequence compensates for any deviation from zero of the addition of theoriginal system. In the present case however—which is also because aneutral conductor is not present or is not taken into consideration—azero sequence is not considered and is thus also not calculated, butonly the positive sequence or the positive sequence component and thenegative sequence or the negative sequence component.

Calculation of a positive sequence and a negative sequence from athree-phase asymmetric system is known to the man skilled in the artfrom text books and in that respect is not described in greater detailhere.

Calculation of the complex voltage values V ₁, V ₂ and V ₃ is based onthe basically known method of discrete Fourier transformation, referredto for brevity as DFT. In a discrete Fourier transformation a periodicsignal is described in unique, that is to say reversible, fashion assuperpositioning of a direct component, a fundamental oscillation andits harmonic. In the simplest case neither a direct component nor aharmonic is present or such components can be disregarded. In that casethe corresponding descriptive components are omitted and a descriptionof the signal in terms of magnitude, phase and frequency is exclusivelyemployed. To perform such a discrete Fourier transformation a periodduration of the periodic signal is to be detected. If a sinusoidalsignal at a frequency 50 Hz is involved, as is the case with theelectric voltage in the European integrated network—in substance thatcan be applied to a 60 Hz network as for example in the USA—then aperiod length is T/=1/f= 1/50 Hz=20 ms. For a discrete Fouriertransformation of the voltage signal of a 50 Hz voltage networktherefore at least 20 ms is required. That time can be very long if theaim is for fast reaction to network faults in the network.

It is now proposed that only half a period length of the signal to betransformed is used. In the present case therefore of each voltagesignal V₁(t), V₂(t) and V₃(t) only half a period length is taken intoaccount in each case. The result of this modified DFT which is alsoreferred to here as half-cycle DFT is calculated in the transformationblock 50 and outputted. Accordingly for each of the three voltage phasesthere is a voltage magnitude V_(i) and a voltage phase φ _(Vi). Thevariable “i” can assume the value 1, 2 or 3 and accordingly denotes the1st, 2nd and 3rd phase respectively.

$V_{i}^{c} = {K_{c}{\int_{0}^{\frac{1}{2{fset}}}{{v_{i}(t)}*{\cos( {2\pi_{fset}*t} )}\ {\mathbb{d}t}}}}$$V_{i}^{s} = {K_{s} - {\int_{0}^{\frac{1}{2_{fset}}}{{v_{i}(t)}*{\sin( {2\pi_{fset}*t} )}\ {\mathbb{d}t}}}}$$\varphi_{\underset{\_}{V}i} = {\arctan( \frac{V_{i}^{s}}{V_{i}^{c}} )}$$V_{i} = \sqrt{( V_{i}^{c} )^{2} + ( V_{i}^{s} )^{2}}$

That calculation is implemented for each phase, which is indicated bythe index i which thus assumes the value 1, 2 or 3 according to therespective phase. Thus firstly a first voltage component V_(i)′ and asecond voltage component V_(i)″ are calculated by means of therespectively specified integral. More specifically therefore a givenintegral of 0 to

$\frac{1}{2\;{fset}}$is calculated. In that case

$\frac{1}{2\;{fset}} = {\frac{1}{2}T}$and thus the given integral is calculated over half a period duration T.A scaling factor K_(c) is also to be taken into account for the firstvoltage component V_(i) ^(c*) and correspondingly a scaling factor K_(s)is to be taken into consideration for the second voltage component V_(i)^(s), wherein those two scaling factors may also be identical. The twointegrals represented can be calculated in different ways. For exampleimplementation of a discrete calculation is also considered, inparticular having regard to the fact that the respective voltage valuesv_(i)(t) are present in a process computer and thus also in thetransformation block 50 in the form of sampling values. Concreteimplementation of such a similar integral formation for example on aprocess computer is familiar to a person of ordinary skill in the art.Moreover it is pointed out that the first voltage component V_(i) ^(c)and the second voltage component V_(i) ^(s) could be interpreted as animaginary part and a real part.

In calculation of the two integrals for the first and second voltagecomponents it is to be noted that the voltage values of v_(i)(t) whichare back by up to half a period duration are respectively taken intoconsideration. In the case of a sinusoidal voltage signal at a frequencyof 50 Hz—to give a practical example—that involves half a periodduration at 10 ms. Accordingly changes approximately after 10 ms arecompletely detected by that modified DFT or half-cycle DFT. The firsteffects however already have such changes when they occur. Thetransformation or calculation of the complex voltage values, namely V ₁,V ₂ and V ₃, proposed in the transformation block 50 in FIG. 3, can thusbe carried out very quickly. The sampling frequency which thetransformation block 50 uses can be for example 5 kHz and thereforethere is a calculation value every 200 μs. That 200 μs is thus theduration after which—in this example—the first effect of a change in thenetwork voltage is reflected in the calculated complex voltage values.

Accordingly after approximately that time there is also an effect inrespect of the values of the positive and negative sequences, that is tosay for V⁺ _(fset), φ _(Vfset), V⁻ _(fset) and φ _(V) ⁻ _(fset).

FIG. 2 shows the further use of the components calculated in thecalculation block 42 for the positive sequence and the negative sequenceas follows:

The state switch 36 is switched in dependence on a fault signal, namelythe flag. If flag=0, that is to say if it is assumed that there is nonetwork fault, the state switch 36 is switched in such a way that thenetwork phase angle φ_(N) which is calculated in the transformationblock 32 is used as an input parameter for the state observer block 34.

If however it is assumed that there is a network fault, then flag=1 andthe state switch 36 switches over so that the phase angle φ _(V) ⁺_(fset), that is to say the calculated phase angle of the positivesequence, forms the input, namely the input angle, of the state observerblock 34. In this case therefore the phase angle of the positivesequence forms the basis for the state observer block 34. That can alsobe interpreted such that at any event reference is made in respect ofthe phase angle to the positive sequence.

The phase angle of the negative sequence φ _(V) ⁻ _(fset) forms an inputsignal of the decision block 38. That angle is required in the decisionblock 38 for the situation where it is assumed that there is a networkfault, as was already explained hereinbefore in connection with thecalculations or processes in the decision block 38. In that respect,upon the assumption of a network fault, the decision block 38 links thephase angles of the positive sequence and the negative sequence and inthat respect takes account of an asymmetry in the network voltages. Asexplained in this case the phase angle of the negative sequence φ _(V) ⁻_(fset) is effected directly and the phase angle of the positivesequence φ _(V) ⁺ _(fset) is effected indirectly by way of stateobservation of the state observation block 34.

The voltage values of the positive sequence V⁺ _(fset) and of thenegative sequence V⁻ _(fset) which were calculated in the calculationblock 42 are used in the PQ-control block 40. The basically desiredcurrent which is to be fed into the network is determined in thePQ-control block, more specifically in respect of all three currents tobe fed in. The determining operation which can also be referred to asthe presetting step accordingly outputs a complex positive sequencecurrent I ⁺ and a complex negative sequence current I ⁻ respectively.Therefore at least the possibility is assumed to exist, namely that thethree-phase current is asymmetric and therefore the description inaccordance with the method of symmetrical components is used. ThePQ-control block 40 admittedly uses the voltage amplitudes V⁺ _(fset)and V⁻ _(fset) which were produced in the calculation block 42 andissue, but for calculation of the positive sequence and negativesequence current components I ⁺ and I ⁻ it implements a dedicatedcalculation, namely breakdown into positive and negative sequences.

The calculation of that predetermined current can take account ofvarious presettings, namely the active power component to be fed in, inparticular the active power component of the positive sequence P⁺ andthe reactive power component to be fed in, namely in particular theactive power component of the positive sequence Q⁺. In addition it ispossible to take account of a ratio of the magnitudes of the current ofthe negative sequence I⁻ to the current of the positive sequence I⁺,namely I⁻/I⁺. That quotient is a measurement of the degree of asymmetryof the three-phase system which is described by that positive sequencecomponent and negative sequence component.

In addition the PQ-control block 40 takes account of fault criteria,from which a network fault can be deduced and generates theabove-described flag which assumes the value 0 if it is assumed thatthere is no network fault and assumes the value 1 if it is assumed thata network fault is present. Such fault criteria can be for example agreat change in frequency, the failure of a phase or also the failure ofor a great reduction in the amplitude of all phases. The fault criterioncan however also be a direct signal which is already the result of anexternal evaluation or which is afforded by a network operator andpossibly in that respect also specifies the nature of the network fault.

The PQ-block can be implemented in different ways. It can for examplesimultaneously take account of V⁺ _(fset) and V⁻ _(fset) and V. Forexample V⁺ _(fset) and V⁻ _(fset), which are basically synthetic values,and V which stands for the real voltage, do not have to be correctlyreproduced. Thus V⁺ _(fset) and V⁻ _(fset) can for example have afrequency fault. The one or the other or both values is used independence on the specific situation.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A method of feeding electric current intoan electric three-phase network comprising first, second and thirdphases with first, second and third voltages at a network frequency, themethod comprising: measuring the first, second and third voltages,transforming the first, second and third voltages into a voltagepositive sequence and a voltage negative sequence using a method ofsymmetrical components, calculating a first, second and third targetcurrents for feeding into the first, second and third phases,respectively, of the electric three-phase network, wherein calculationof the first, second and third target currents is effected in dependenceon at least one value of the voltage positive sequence and the voltagenegative sequence.
 2. The method according to claim 1 wherein electricenergy is produced by a wind power installation and fed into theelectric three-phase network.
 3. The method according to claim 1 whereinelectric currents corresponding to the first, second and third targetcurrents for feeding into the electric three-phase network are producedby a frequency converter and fed into the electric three-phase network.4. The method according to claim 1 wherein calculating the first, secondand third target currents is based on a calculation phase angle that isdetermined in dependence on detection of a network fault using adetermination filter or filter block, wherein the calculation phaseangle is determined from a detected phase angle of one of the networkvoltages when no network fault is detected; and the calculation phaseangle is determined from a phase angle of the voltage positive sequenceusing a predetermined network frequency when a network fault isdetected.
 5. The method according to claim 1 wherein calculating thetarget currents reference is made to the voltage positive sequence,including the phase angle of the voltage positive sequence.
 6. Themethod according to claim 1 wherein transforming the first, second andthird voltages into the voltage positive sequence and the voltagenegative sequence includes transformation of the first, second and thirdvoltages by a discrete Fourier transformation, wherein the discreteFourier transformation is effected online and over half a periodduration.
 7. The method according to claim 1 wherein transforming thefirst, second and third voltages into the voltage positive sequence andthe voltage negative sequence uses a predetermined frequency instead ofusing the currently prevailing network frequency.
 8. The methodaccording to claim 7 wherein the predetermined frequency is one of avalue of the network frequency of an earlier measurement time, a nominalvalue of the network frequency, and an externally predetermined value.9. The method according to claim 1 wherein the first, second and thirdtarget currents are predetermined using the method of symmetricalcomponents by way of a current positive sequence and a current negativesequence, and wherein the positive sequence is taken into considerationby a complex positive sequence current component with respect tomagnitude and phase and the negative sequence is taken intoconsideration by a complex negative sequence current component withrespect to magnitude and phase.
 10. The method according to claim 9wherein the first, second and third target currents are calculated independence on the current positive sequence or the positive sequencecurrent component and assumes a network fault.
 11. The method accordingto claim 9 wherein the current positive sequence or the positivesequence current component and the current negative sequence or thenegative sequence current component is determined in dependence on atleast one of: a presetting of an active power component of the positivesequence, a presetting of a reactive power component of the positivesequence, and a presetting of a quotient of the magnitude of thenegative sequence current component in relation to the magnitude of thepositive sequence current component.
 12. The method according to claim 9wherein the negative sequence current component is set or variedindependently of the positive sequence current component.
 13. The methodaccording to claim 1 wherein the method is carried out online.
 14. Awind power installation which uses a method according to claim 1 forfeeding electric current into the electric three-phase network.
 15. Awind park comprising a plurality of wind power installations with atleast one wind power installation according to claim 14.